1. Field of the Invention
The invention relates to an asymmetric polyphase filter comprising first to fourth input terminals for applying thereto a 4-phase input signal comprising first to fourth signal vectors, respectively, which succeed one another in phase each time through 90.degree., first to fourth intercoupled identical filter sections, respectively, connected to said terminals, at least one of the odd and even filter sections being coupled to two output terminals, said filter sections having constant reactances.
2. Description of Related Art
Such an asymmetric polyphase filter is known from the Article "Asymmetric Polyphase Networks" by M. J. Gingell, published in "Electrical Communication", Vol. 48, no. 1 and 2, 1973, pp. 21-25.
The known polyphase filter has an N number of mutually identical filter sections which are incorporated between an N number of inputs and an N number of outputs. The filter sections are interconnected at certain points and constitute a physically symmetrical network, i.e. the electrical paths from each input to the corresponding output are mutually identical. A polyphase or N-phase input signal is applied to the inputs of the polyphase filter, which signal, if for example N=4, may consist of four signal voltages of mutually equal value jointly constituting a signal vector group whose four signal vectors succeed one another in a given direction of rotation through phase angles of 90.degree. each. Dependent on this direction of rotation--counter-clockwise or clockwise--the frequency of the polyphase signal is positive or negative, or conversely. The asymmetric polyphase filter has an asymmetrical frequency transfer characteristic H(.omega.), i.e. the filter transfer for negative frequency values (.omega.&lt;0) of the polyphase input signal deviates from that for positive frequency values (.omega.&gt;0). To this end each filter part has one or more constant reactances, i.e. positive or negative frequency-independent imaginary conductances. To eliminate restrictions in the possibility of choosing the shape of the asymmetrical frequency transfer characteristic to a maximum possible extent, each of these constant reactances is realized by means of an N-port gyrator. However, this leads to a comparatively complex circuit configuration which is difficult to realize in a circuit symmetry which is sufficiently accurate for a correct filter operation.